Electronic apparatuses are known in which a plurality of electric structures are bonded to each other using an electroconductive adhesive containing a thermosetting resin and a metal powder.
For example, Patent Document 1 proposes a connected structure in which, as shown in FIG. 6, a first electrode 102 is disposed on a substrate 101, a second electrode 104 is disposed on an electronic device 103, the first electrode 102 and the second electrode 104 are connected to electrode-connecting parts 105 in which conduction is ensured by fusion-bonding of metal fine particles, an intermediate connecting part 106 is interposed between the electrode-connecting parts 105, the metal fine particles are fusion-bonded to each other at a temperature equal to or lower than the thermosetting temperature of the electroconductive adhesive, and the electroconductive adhesive contains an electroconductive filler having a particle size that does not allow the particles to be fusion-bonded to each other at a temperature equal to or lower than the thermosetting temperature of the electroconductive adhesive.
In Patent Document 1, the electrode-connecting parts 105 are composed of an electroconductive adhesive containing metal fine particles, such as Ag, that are fusion-bonded to each other at a temperature equal to or lower than the thermosetting temperature of the electroconductive adhesive, an electroconductive filler having a particle size that does not allow the particles to be fusion-bonded to each other at a temperature equal to or lower than the thermosetting temperature of the electroconductive adhesive, and an adhesive.
Furthermore, in Patent Document 1, by means of heat-hardening treatment, the first electrode 102 and the second electrode 104 are fusion-bonded to the electroconductive filler through the metal fine particles contained in the electrode-connecting parts 105, and the electroconductive filler particles are fusion-bonded to each other through the metal fine particles, thereby improving adhesion at interfaces.
Patent Document 2 proposes a thermally conductive bonded member in which, as shown in FIG. 7, a first substrate 107 and a second substrate 108 are bonded to each other through a thermally conductive material 109.
The thermally conductive material 109 proposed in Patent Document 2 includes a thermosetting resin 110 containing an organic acid, and a thermally conductive filler. The thermally conductive filler includes a first filler 111 having a melting point higher than the thermosetting temperature of the thermosetting resin 110 and a second filler 112 having a melting point lower than the thermosetting temperature of the thermosetting resin 110.
Furthermore, Patent Document 2 discloses an example in which preparation was made such that the volumetric ratio of the thermally conductive filler (the first and second fillers) to the total volume of the thermosetting resin 110 and the thermally conductive fillers 111 and 112 was 50% by volume (corresponding to 89% by weight).
In Patent Document 2, since the thermally conductive filler includes the second filler 112 having a melting point lower than the thermosetting temperature of the thermosetting resin 110, the second filler 112 is melted before the thermosetting resin 110 is hardened, and the first filler 111 and the second filler 112 are fusion-bonded to each other. Furthermore, since the thermally conductive filler includes the first filler 111 having a melting point higher than the thermosetting temperature of the thermosetting resin, the first filler 111 maintains its form even after the thermosetting resin 110 is hardened, and thereby, an increase in thermal resistance can be suppressed. Moreover, since the thermally conductive material includes the thermosetting resin 110 and the thermally conductive filler, the bonding temperature can be decreased, and also, since the thermally conductive material contains a resin-based material as a base, the elastic modulus is low, and thermal stress can be decreased. In this way, it is intended to obtain a thermally conductive material having high thermal conductivity and good connection reliability.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-93826
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-335872
However, in Patent Document 1, although good conduction properties are ensured in the electrode-connecting parts 105 by fusion-bonding of metal fine particles that are melted at a temperature equal to or lower than the thermosetting temperature of the electroconductive adhesive, the conducting properties are poor in the intermediate connecting part 106 because conduction properties are ensured only by contact between the electroconductive filler particles, which is a problem.
In order to avoid such a problem, a method is conceivable in which low-melting-point metal powder that is melted at the thermosetting temperature of the electroconductive adhesive is incorporated into the electroconductive adhesive.
However, in the case where a bonded structure in which a plurality of electric structures are bonded to each other is mounted on a motherboard or the like, reflow heating treatment may be repeatedly performed, and thermal shock accompanied with a rapid temperature change may be applied to the bonded structure. Thus, there is a possibility that the connected structure may be exposed to a high-temperature atmosphere for a long period of time. This may cause remelting of the low-melting-point metal powder, resulting in difficulty in obtaining sufficient bonding strength. Moreover, in such a case, when melting and hardening are repeated due to thermal shock, etc., there is a possibility that separation may occur at the bonding interfaces between the electrode-connecting parts 105 and the intermediate connecting part 106.
In particular, in the reflow heating treatment, it is becoming common to use Pb-free solder in consideration of the environmental aspects, etc. The Pb-free solder has a high melting temperature of 270° C. to 280° C. Consequently, the hardened metal may be remelted, and separation may be more likely to occur at bonding interfaces.
Furthermore, in Patent Document 2, the content of the thermally conductive filler and the particle sizes of the first filler and the second filler are not taken into consideration. That is, depending on the content of the thermally conductive filler or the particles sizes of the first filler and the second filler, bonding strength with respect to heat-hardened bonding surfaces may be decreased. Furthermore, when exposed to high temperature and high humidity conditions for a long period of time, connection resistance may be increased, and conduction properties may be degraded.